Source Kitchen Research Articles

ONSHORE AND OFFSHORE PETROLEUM SEEPAGE: CONTRASTING A CONVENTIONAL STUDY IN PAPUA NEW GUINEA AND AIRBORNE LASER FLUOROSENSING OVER THE ARAFURA SEA

Two case studies from the northern margin of the Australian continental plate are presented to illustrate mapping and understanding seepage in an onshore and an offshore area. The first involves an integrated approach to the detection and analysis of onshore seeps in the Aure Thrust Belt of Papua New Guinea. The second study is an application of BP's proprietary Airborne Laser Fluorosensor (ALF) technology to the systematic detection and mapping of offshore oil seepage in the Arafura Sea, northern Australia.In the absence of well or outcrop source rock data, the source risking in the Aure Thrust Belt has been constrained using oil and gas seeps. Mapping the seeps required optimum use of the local peoples' observational prowess. Geochemical analyses of the seeps allowed the identification of an oil-prone source rock of probable Jurassic age. The data reveal that the seeping petroleum liquids are gas condensates in the subsurface, and provide information on the maturation history of the source and the nature of the reservoir/carrier beds, as well as differentiating the biogenic and thermogenic gas components. In this uplifted region, seepage is likely to be from accumulations only.To help reduce the exploration risk in offshore areas such as the Arafura Sea, BP have developed the ALF system which detects oil seepage at the sea surface. It does this by detecting its characteristic fluorescence, which is induced by an aircraft-mounted ultraviolet excimer laser.The ALF data over the western Arafura Sea indicate active oil seepage showing a distribution which is compatible with our current understanding of the subsurface Palaeozoic and Mesozoic source kitchens. The mapped seepage over the Goulburn Graben is most likely derived from the Palaeozoic to (?)Triassic section (with a possible contribution from other Mesozoic sources), and must be migrating through a largely unfaulted Mesozoic seal. Additionally, there is evidence for liquid petroleum seepage from the Mesozoic section in the Calder Graben via faults which cut through the regional seal along the Lynedoch Bank Fault System. In this region of the Arafura Sea, seepage is likely to derive from accumulations, mature source kitchens and secondary migration routes.

The organic geochemistry of petroleum reservoirs

The organic constituents of petroleum reservoirs frequently show measurable compositional variations both laterally (between wells) and vertically (within a single well). The pattern of variation is controlled by the geological setting of the reservoir, which influences the way in which migrating petroleum fills the reservoir from one or more source kitchens. Having achieved a high petroleum pore saturation, the reservoir fluids may then redistribute themselves at rates are often comparable to geological time periods. Mass transfer by diffusion and density-driven convection must be considered, and their timescales compared with the elapsed time since the accumulation was filled. In many cases, the compositional variations “inherited” from the filling process persist to the current day. This paper outlines the mechanisms by which reservoirs fill and mix, and describes how this can lead to observable lateral differences in composition. The relationship between certain engineering parameters (particularly the gas oil ratio and reservoir fluid density), reservoir (rock) description and geochemical parameters will be stressed. The goal being to improve the characterisation of petroleum reservoirs by combining geochemical and engineering data into a scientifically consistent framework. Many aspects of the geochemistry of petroleum reservoirs have only recently begun to be researched. It is hoped that the recent advances in the geochemical characterisation of oils and gases will be increasingly applied to the problems of reservoir and petroleum engineering. This should lead to improvements in the efficiency with which petroleum is recovered from the subsurface.